[DevTools] Improve error checking on custom fields for HAR imports
[DevTools] Improve error checking on custom fields for HAR imports

Bug: 968623
Change-Id: Ie0aaa7bc036ea5f170270f0f2e26ad158381b440
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1639317
Reviewed-by: Erik Luo <26192f5e05d23e5ee959014fb911d780e566545b@chromium.org>
Commit-Queue: Joey Arhar <b82f612a3ddcfbac27d1758ce0a3eba47bee1116@chromium.org>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#665318}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 9c907b623389b7e87c21da3276cca1ff1f0b9116

ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend